The Influence of Defects on the Luminescence of Trivalent Terbium in Nanocrystalline Yttrium Orthovanadate

Abstract

Terbium-doped YVO 4 has been considered a nonluminescent solid since the first classic studies on rare-earthdoped phosphors in the 1960s. However, we demonstrate that defect engineering of YVO 4 :Tb 3+ nanoparticles overcomes the metal−metal charge transfer (MMCT) process which is responsible for the quenching of the Tb 3+ luminescence. Tetragonal (Y 1−x Tb x )VO 4 nanoparticles obtained by colloidal precipitation showed expanded unit cells, high defect densities, and intimately mixed carbonates and hydroxides, which contribute to a shift of the MMCT states to higher energies. Consequently, we demonstrate unambiguously for the first time that Tb 3+ luminescence can be excited by VO 4 3− → Tb 3+ energy transfer and by direct population of the 5 D 4 state in YVO 4 . We also discuss how thermal treatment removes these effects and shifts the quenching MMCT state to lower energies, thus highlighting the major consequences of defect density and microstructure in nanosized phosphors. Therefore, our findings ultimately show nanostructured YVO 4 :Tb 3+ can be reclassified as a UV-excitable luminescent material.

Simple Postsynthesis Thermal Treatment toward High Luminescence Performance of Rare Earth Vanadate Nanoparticles

“…14,42,43 Moreover, the luminescence of Tb 3+ in YVO 4 also decreases under UV irradiation due to the partial oxidation of vacancies in air. 26 We therefore compared the time evolution of Eu 3+ luminescence in pristine and annealed YVO 4 nanoparticles under constant UV LED irradiation (λ exc = 280 nm, I = 4.4 mW cm −2 ), monitoring both particles as suspensions in phosphate-buffered saline (PBS, Figure 4a) and as powders deposited on nitrocellulose membranes (Figure 4b). While asprepared colloidal particles retained only 37 ± 4% of the initial luminescence intensity after 5 min of light exposition, solids annealed at 800 and 1000 °C kept 70 ± 4% and 87 ± 5% of the initial signal, respectively.…”

“…Comparing the dimensions of the particles recovered after treatment at 1000 °C and the corresponding crystalline coherence lengths (133 ± 25 nm Figure g), we demonstrate that the protocol yielded nearly monocrystalline nanoparticles with faceted planes and reduced microstrain [(1.9 ± 0.9) × 10 –4 , Table S1]. The annealing also impacts lattice parameters, where pristine particles showing expanded unit cells due to surface defects ,− undergo progressive unit cell constriction after heating at 800 and 1000 °C (Table S1). Both pristine and annealed particles showed an even Y/V/Eu elemental distribution (STEM/EDS, Figures S10–S12), also confirming the compositional and phase homogeneity of the products.…”

“…Despite the common comparison with the photobleaching of organic fluorophores to affirm that lanthanide emissions are stable toward UV irradiation, the complex redox and defect chemistry of some oxide nanoparticles together with interactions with the solvent may also cause progressive time-dependent quenching of Ln 3+ emissions . For instance, Eu 3+ -doped REVO 4 aqueous colloids show a decrease in luminescence under UV excitation due to the reduction of the host matrix (V 5+ /V 4+ ) or the reduction of the activator ion (Eu 3+ /Eu 2+ ). , A reversible photoreduction of Eu 3+ upon direct Eu 3+ excitation or a reversible chemical reduction of GdVO 4 :Eu 3+ particles was observed and exploited by us to implement reactive oxygen species-sensitive nanosensors. ,, Moreover, the luminescence of Tb 3+ in YVO 4 also decreases under UV irradiation due to the partial oxidation of vacancies in air …”

Nanoporous YVO4 as a luminescent host for probing molecular encapsulation

“…Highly basic pH results in higher concentrations of VO 4 3À (aq), but also induces competitive precipitation of kinetically stable RE hydroxides. 14,15 Conversely, acidic metavanadate ((VO 3 ) n nÀ ) solutions show a smaller availability of free VO 4 3À and may lead to the parallel precipitation of polyvanadates. 16 We therefore evaluated initially the effect of the pH (= 7, 11.5 or 13) of the aqueous cores on the interface crystallization of REVO 4 in reverse microemulsions with W O = 25 (ESI, † Section S2.1 and Fig.…”

“…Highly basic pH results in higher concentrations of VO 4 3− (aq), but also induces competitive precipitation of kinetically stable RE hydroxides. 14,15 Conversely, acidic metavanadate ((VO 3 ) n n − ) solutions show a smaller availability of free VO 4 3− and may lead to the parallel precipitation of polyvanadates. 16…”